The invention relates to a method of performing dynamic mechanical analyses, wherein a specimen under investigation is held in a holder device and is subjected to a static pre-tensioning force and a time-variable excitation force generated by an excitation device, and wherein the deformation of the specimen is measured by means of one or more displacement sensors. The invention further relates to an apparatus for performing the method. Included in the apparatus are a controller device that directs and controls the analysis process, a holder device for holding the specimen, an excitation device that allows a static pre-tensioning force and a time-variable excitation force to be applied to the specimen, and one or more displacement sensors to measure the deformation of the specimen.
Dynamic mechanical analyses (DMA) are used to determine visco-elastic material properties. To perform a dynamic mechanical analysis, the samples to be investigated are clamped in specimen holders or, in more general terms, connected to suitable holder devices, and stressed with a dynamic force. In order to determine physical quantities from the DMA test, a time profile is registered of the displacement of the specimen and the excitation force acting on the specimen. Of particular interest among the data collected for the excitation force and the displacement are the ratio between excitation force and displacement as well as the phase lag between the two variables. Taking into account the specimen dimensions and the specifics of the excitation arrangement, this information can be used to determine the components of the complex stress/strain tensor or, in other words, to determine an elastic component and a viscous component of the modulus of elasticity. The dynamic mechanical analysis characterizes the visco-elastic behavior, for example under the influence of temperature, different excitation frequencies, phase changes, or chemical transformation of the specimen. To perform temperature-dependent measurements, the specimen is arranged in a test compartment in which the temperature can be varied.
Methods and apparatus for dynamic mechanical analyses are known, e.g., from U.S. Pat. Nos. 5,710,426, 6,058,784, EP 0078373A1, EP 1126269A1, DE 4229549A1, DD 222120A1, U.S. Pat. No. 6,098,465 and EP 0921388A2. The state-of-the-art systems include at least a specimen holder or holder device for a test specimen, an excitation device, and a displacement sensor. The excitation device is connected to an excitation part of the holder device. An anchoring side of the holder device is connected to a stationary part of the measuring apparatus. The excitation force acting on the test specimen can be determined, e.g., by a measurement with a force sensor, or from the voltage and/or current supplied to the excitation device. As a result of the excitation force, the specimen exhibits a deformation that is measured by the displacement sensor, which is arranged at the excitation side of the holder device.
In situations where test samples are subjected to both a base load (i.e., a static force) and a dynamic force, the results have errors if an inadequate base load is selected. For example, a tensile test specimen in an experiment to investigate a correlation between a tensile force and an elongation can be clamped in place with a slack so that, as a result, the dynamic force will not provide an effective elasticity measurement of the test specimen because at least a part of the dynamic force is used to put the specimen completely under tension. The clamping of the test specimen has an analogous effect on the measurement in the case of compressive or bending test samples. The amount of pre-tension required for the desired type of coupling of the test specimen to the excitation device can change during a measurement process, for example if the dimensions of a specimen change because of a temperature change.
If in a dynamic mechanical analysis of a polymer the temperature is raised towards the glass transition temperature of the polymer, it is possible for the polymer material to soften by a factor 1000. When the measurements approach the glass transition temperature, an amount of pre-tension that was required at low temperatures will produce such a strong deformation of the specimen that the measurements can be made only within a limited temperature range. Thus, it is impossible in a range of particular interest, i.e., in a critical temperature range approaching the glass transition temperature, to perform continuous measurements if the same amount of pre-tension is used that is required at a low temperature. In addition to the problems that occur with samples that become softer or expand as a result of a temperature change, there are also measurement problems associated with test samples that shrink under a temperature change.
In the known state of the art of dynamic mechanical analysis devices, the amount of pre-tension used may in some cases be too small, or it may be unnecessarily large. No efficient way is known for monitoring the pre-tension in the sense of ensuring that the test specimen is fully pre-tensioned or, in other words, coupled correctly (i.e., without slack) to the excitation device. The errors introduced into the measurement by an insufficient amount of pretension remain unaccounted for or are ignored. Because the physical measurement values that apply to a correctly coupled specimen can be different at a higher amount of pre-tension, the measurement can also be impaired by an excessive amount of pre-tension.